Electrical contact brush



United States Patent 3,165,480 ELECTRICAL CONTACT BRUSH 7 Ronald W. Churchill, Jr., Fostoria, Ohio, assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed Dec. 18, 1957, Ser. No. 703,513

laims. (Cl. 252--503) This invention relates generally to electric contact brushes for use as current-collectors or distributors in dynamo-electric apparatus and refers more particularly to carbon or graphite brushes of the type incorporating film depositing additives.

It has been found that atmospheric water vapor and oxygen together deposit a film on the commutator of dynamo-electric apparatus which is advantageous for lubricating the commutator surface and enhancing the wear life of the brushes. When the paucity of water vapor or oxygen in the atmosphere is critical, so that the brushes operate in an extremely dry or rare atmosphere, the brushes wear extremely fast, or dust. This dusting occurs under high altitude conditions and results in very short brush life. I

In the past, the problem of providing the film under rare ordry atmospheric conditions has been circumvented by various means. Among the most common and successful were the insertion of film-forming materials in the form of plugs or inserts into the brush, the use of film-forming materials in powdered form in the brush body where the powders were mixed with the brush stock before forming and the use of auxiliary bearing surfaces much like brushes but made up wholly of some filmforming material. Brushes of the inserted plug type had the advantage over powder-containing brushes in that they provided commutator filming and lubrication at high altitudes, as did the latter type brush, and also prohibited over-filming at low altitudes or under conditions of normal humidity under which conditions the powder-containing brushes over-filmed, due to the abundance of moisture. However, the inserted plug type brushes have the disadvantage or fault of seriously grooving the contact surface and of depositing a film only on a localized area of surface with theresul-t that the film was not uniform.

It is therefore an object of this invention to provide an improved electrical contact brush of the type capable of adjusting itself to operation either under normal or adverse conditions of humidity.

A further object of this invention is to provide an improved electric contact brush of this type containing discrete particles of an inorganic lubricant so disposed in the body of the brush that uniform filming and minimum surface wear of the contact surface is possible.

Further objects and advantages of this invention will become apparent and this invention will be better understood from the following description.

The invention comprises the provision of discrete particles of a film-forming or lubricating material or mixture of materials, which material or mixture preferably contains a metal or metallic compound, uniformly distributed throughout the entire brush mass. The particles preferably contain a metal or metallic compound so as to make the film-forming particles electrically conductive. Such metals as copper, lead, and silver may be used for this purpose. Examples of compound film-forming materials that may be employed successfully are barium fluoride, molybdenum, disulphide, lead oxide, zinc sulphide, silver sulphide and calcium carbonate. The preferred materials of the invention however are molybdenum disulphide and metallic silver. The total weight percent of the filmforming particles in the entire brush mass should be between 5 percent and 30 percent, with an optimum of percent. The particle dispersion should be substantially uniform throughout the entire body of the brush so that a weight percentage of particles within the broadest range will result in 7 percent to 8 percent of the bearing surface of the brush being made up of the additive particles at any given time. This invention shows that this percentage and distribution gives good commutation and filming performance with minimum surface wear.

One method of making the article of the invention, the brush, is by blending silver and molybdenum disulphide powder, compacting the mixture, crushing it and then screening it to a desired particle size. These particles are then dry blended with a milled mix, which may consist of pitch-graphite or pitch-carbon, and the entire blend is compressed into the appropriate size and baked.

As a specific example of the practice of the invention,

the following may be cited. Equal parts by weight of powdered silver, having a particle size such that about 82% passed through a 325 mesh screen, and molybdenum disulphide (96 percent pure molybdenite powder), sized so that about 62% passed through a 325 mesh screen, were blended and the blended mixture was compressed, under about 15 tons per square inch pressure, into blocks 1% inches by 2% inches by /2 inch in size. These blocks were then crushed into inch to 1 inch pieces, and then coarsely milled. The desired particles which were of a size sufficient to pass through a 35 mesh screen and rest on a 65 mesh screen, were separated from the coarsely milled material by screening (all screen sizes are Tyler standard). A similar pitch-graphite milled mixture was prepared by mixing, at about 165 C.: 100 parts by weight of graphitized coke flour, percent through 200 mesh, and 56 parts by weight of hard coal tar pitch having a melting point of about C., cooling the'rnixture to room temperature and milling to a size sufiicient to pass 100 percent through 35 mesh. A dry blend of 80 percent of the milled mixture and 20 percent film depositing particles was then prepared by tumbling the components for a time sufficient to ensure uniformity of composition, whereafter the blend was compressed, under about 15 tons per square inch pressure, into blocks 1%. inches by 2% inches by /2 inch in size. The blocks were'then packed in finely milled petroleum coke and baked to 900 C.

Experiments indicate that by uniformly distributing discrete particles of the film-forming material throughout the brush mass, over-filming is eliminated and because of the distribution of the material within the brush mass there is no localized lubrication or grooving of the commutator. It has been found that this uniformdispersion of the discrete particles provides a situation where, as the brush wears away new particles appear on the face thereof in varying position but in the same proportion. This fact makes localized grooving and lubrication, as is found in the inserted plug type brush, impossible because of the constantly changing position of the film forming and lubricating par-ticles. Further the particles show vast improvement over powder additives since, because of size, there is no over-filming under conditions of normal humidity and altitude. This invention further shows that the discrete particles should be no larger than 10 mesh but larger than 100 mesh with an optimum range being larger than 65 mesh but not larger than 35 mesh.

The advantages of the use of this invention may be readily seen from an analysis of various tests which have been conducted. The brushes used for the tests were those made in accordance with the invention and those of the prior art. Test data are reported in Table I below. All brushes tested, except those tabulated as numbers 3 and 8 in which the film-forming materials were present in powder form, were brushes embodying the invention in which the filmforming materials were present in the form of discrete particles. The screen analysis of the film-forming materials incorporated in the brushes are reported in the table. The expression 35 +65, for example, means that all particles passed through a 35 mesh screen but were retained on a 65 mesh screen. The relative friction of each brush was determined by arbitrarily' designating brush number 3 as 1 and relating the friction of the other tests to this one.

All tests were made using slip rings which were run at a speed of about 5200 surface feet per minute with a brush pressure of about 6 pounds per square inch. In the tests run at sea level, the current load through the test brushes was 40 amperes and the current density was 122 amperes per square inch. The high altitude tests (40,000 feet) were run with a 20 ampere current load and a current desity of 61 amperes per square inch.

Table I TESTS AT SEA LEVEL Percent Brush Additive Screen Relative Contact Temper- No. Analysis Friction Drop MOS; Ag

TESTS AT SIMULATED ALTITUDE OF 40,000 FEET It may readily be seen from these data that, under both sea level and altitude conditions, particle-containing brushes, made in accordance with this invention, had generally lower relative friction, contact drop and brush operating temperature than did the corresponding brushes which contained powdered additives. Other tests were conducted under sea level conditions, ona brush identical with brush number 1 of Table I and a brush of the same brush stock but containing a plug, composed of 50% molybdenum disulfide and 50% silver, instead of discrete particles. These brushes wer held by a pressure of about 6 pounds per' square inch on a commutator turning at about 5200 feet per minute. After equal periods of friction, the commutator surface, on which the brushes had been running, was measured for wear. That surface on which the particle-containing brush had been running showed wear at the rate of 2.1 l0 inches per hour, while that surface on which the plug-containing brush had been running showed a wear rate of 1.66 inchesper hour. The plug-containing brush wore the commutator surface about 8 times faster than did the particle-containing brush.

While particular embodiments of this invention have been herein disclosed, modifications of these will occur to those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular arrangements disclosed. For example, other forms of carbonaceous base materials have been used successfully in the practice of the invention instead of the artificial graphite base materials herein mentioned. Of those employed successfully, natural graphite, calcined lampblacks, raw lampblacks, and combinations thereof have been used. Their use, it will be understood, as well as other carbonaceous base materials will often depend upon the specific application of the brush of the invention.

What is claimed is:

,1. A carbon electrical contact brush consisting of from 5 to 30 weight percent of discrete particles containing at least one film-forming and lubricating material selected from the group consisting of barium fluoride, molybdenum disulphide, lead oxide, zinc sulphide, silver sulphide and calcium carbonate, and a metal selected from the group consisting of silver, copper and lead in an amount sufficient to render said particles electrically con ductive; said metal being present in an amount up to 25 weight percent of said brush; said particles being of a size that will pass through a 10 mesh Tyler standard screen and rest on a 100 mesh screen and being substantially uniformly distributed throughout the entire brush mass, the contact surface of said brush being composed of 7 to 8 percent by area of said particles initially and as the brush wears, theremainder of said brush being carbon.

2. A brush'as described in claim 1 wherein said filmforming and lubricating material is molybdenum disulphide.

3. A brush as described in claim 1 wherein said metal is silver.

4. A brush as described in claim 1 wherein said filmforming and lubricating material is molybdenum disulphide and said metal is silver.

5. A brush as described in claim 4 wherein said particles are of asize that will pass through a 35 mesh an rest on a mesh Tyler standard screen.

References Cited in the file of this patent UNITED STATES PATENTS Arnold Apr. 25, 1961 

1. A CARBON ELECTRICAL CONTACT BRUSH CONSISTING OF FROM 5 TO 30 WEIGHT PERCENT OF DISCRETE PARTICLES CONTAINING AT LEAST ONE FILM-FORMING AND LUBRICATING MATERIAL SELECTED FROM THE GROUP CONSISTING OF BARIUM FLUORIDE, MOLYBDENUM DISULPHIDE, LEAD OXIDE, ZINC SULPHIDE, SILVER SULPHIDE AND CALCIUM CARBONATE, AND A METAL SELECTED FROM THE GROUP CONSISTING OF SILVER, COPPER, AND LEAD IN AN AMOUNT SUFFICIENT TO RENDER SAID PARTICLES ELECTRICALLY CONDUCTIVE; SAID METAL BEING PRESENT IN AN AMOUNT UP TO 25 WEIGHT PERCENT OF SAID BRUSH; SAID PARTICLES BEING OF A SIZE THAT WILL PASS THROUGH A 10 MESH TYLER STANDARD SCREEN AND REST ON A 100 MESH SCREEN AND BEING SUBSTANTIALLY UNIFORMLY DISTRIBUTED THROUGHOUT THE ENTIRE BRUSH MASS, THE CONTACT SURFACE OF SAID BRUSH BEING COMPOSED OF 7 TO 8 PERCENT BY AREA OF SAID PARTICLES INITIALLY AND AS THE BRUSH WEARS, THE REMAINDER OF SAID BRUSH BEING CARBON. 